Synthesis and bio-activity of complex azo compounds: A review on formazans and its complexes

Abstract

Formazans are well-known complexing agents and form highly stable complexes with verity of metals that are useful for number of pharmaceutical applications. The activity profile of formazans and their complexes are huge that include antibacterial, antifungal, antioxidant, anti-fertility, anti-tubercular, antiviral, anti-inflammatory, anticancer, anti-HIV, and anti-proliferative are among the biological activities covered by the formazans. The structures, synthesis, reactions, and spectral properties of formazans have been studied to highlight their potential applications in a variety of bioactive phenomena and analytical applications. The biological potential of the compounds will be greatly enhanced when substitution is carried out at R and R₂. We observed that electron withdrawing groups at para position (if R, R₁ & R₂ = phenyl ring) are the best substitutions to achieve potent pharmacological activities.

1 Introduction

Formazan belongs to a significant class of organic molecules with two distinct functional groups on the same compound azo group(-N = N-) directly linked to imine group(-C = N-)[1]. Formazan compounds are highly colored due toΠ-Π^* electron transfiguration in conjugated double bonds present among azo and hydrazo moieties [2]. Von Pechmann [3] and Bamberger [4] independently described their structure for the first time in 1892.

The chemical and biological activities of formazans are conflicted by tautomerism, particularly their chelating and color properties. The tautomerism in formazans was initially described by Pechmann and Runge [3] in 1894, although their findings were unsatisfactory. Hunter and Robert [5] (1941) demonstrated convincingly that the particular in each duo were identical for multiple pairs of formazans, despite previously being classified as tautomeric (2 & 3). Formazans were supposed to be hydrogen-bond resonance composites with a chelated construction. Accordingly, Hunter and Robert suggested an integrally regulated hydrocarbon arrangement which was demonstrated by two mesomeric forms (4 & 5). As a result, the formazan molecule seems to be a resonance hybrid of these two conformations [2]. The tautomeric (2 & 3) and mesomeric (4 & 5) forms of formazans have been shown in Scheme 1.

Scheme 1

a) Tautomerism in formazan (2 & 3); b) Mesomeric forms of formazan (4 & 5).

A formazan molecule acknowledged the persistence of four different configurations which were encountered by geometrical stereoisomerism around the two double linkages (C = N, syn-anti and N = N, cis-trans). The nomenclature syn/anti is used to describe the orientation around the (C = N) bond, while cis/trans denotation is used to represent the (N = N) bond. Ultimately, an exchange of the (C-N) interactions stated as s-cis or s-trans [6]. The alternatives to minimizing tautomerism for instant are the following (Scheme 2).

Scheme 2

Geometrical forms 6–9 of Formazan.

Formazans containing triphenyl rings are typically very soluble in organic solvents; their dissolution rate appears to be minimal in water because of the pigments of the solvent. The physical and molecular properties of these chromogenic compounds are heavily influenced by their terminal groups [7]. When formazans are oxidized, they produce tetrazolium salt. The formazan pair is a redox mechanism that functions as a proton receiver or oxidant in a range of natural sciences, including pharmacy, botany, toxicology, and immunology, although exclusively in biochemistry and cytochemistry [8, 9].

Their implementation in the chemistry field, due to its global scopes in pharmaceutical, medical, industrial and, chemical fields and synthesis of heterocyclic compounds has influenced the notice of many groups of the researcher [10]. Anti-fertility and anti-malaria activities have been shown by several formazans [11]. Tetrazolium salts are categorized as vitality promoters, and formazans have been discovered to have essential medical applications [12]. Owing their remarkable pharmacological and therapeutic efficacy as an anti-fertility [13], antibacterial and antifungal [14], antiviral [15], anti-microbial [16], anti-HIV and anticancer [17], anti-inflammatory agents [18], anti-tubercular agents [19], formazans and related functionalized molecular structure have received much attention. There are few publications on formazans and their derivatives for biomedical practices, with the majority of them focusing on coordination complexes of formazan [20]. Also, their derivatives and conjugates have recently been shown to possess promising anticancer action in cancer cells using photodynamic treatment [21]. The formazans, on the other hand, are more practical in determining the efficacy of anti-cancer medications [22] to determine tumor cell activity [23] and sperm viability. For the detection of paracetamol, a formazan derivative was used as an electrochemical sensor to modify a pencil graphite electrode [24].

1.1 Historical background

In histochemistry, formazans are thought to be made from tetrazolium salts [25], but this is not the case; in reality, tetrazolium salts are made chemically by oxidation of the matching formazan. Friese [6] (1875) was the first to synthesize a formazan; however, he was unaware of the structure because this category of the compound was previously unknown. Von Pechmann [3] and Bamberger [4] (1892), independently identified the structure of the related compounds, and proposed the name “formazyl.” Von Pechmann and Runge [26] (1894) synthesized triphenyl tetrazolium chloride after studying the oxidation products of the formazan compounds. Nineham [5] (1955) listed 414 formazans that had been described and the current figures are probably more than 1000.

1.2 Motivation and purpose of study

Formazan derivatives are well-known complexing agents so lots of work have been done in this area which will be summarized by the researchers from time to time. Nineham [5] was the first that summarized the chemistry of formazans and tetrazolium salts in the year 1955. Further, in 1976, Altman [27] described the properties of formazans and illustrated how the tetrazolium salt-formazan reaction has been exploited to serve an extremely wide variety of functions. Furthermore, in years 1991, 2006, and 2019 all the information about the design and chemistry of formazans were compiled by Seidler [28], Sigeikin et al. [29] and Lipunova et al. [30]. However, in the year 2016 Shawali and Samy [31] summarized the pharmacological activities of formazans in well-decorated manner. Recently, in the year 2020, Gilroy and Otten [32] summed up details regarding formazan metal complexes in a very impressive way. After evaluating all the studies, the authors observed that there are no epitomized reports which cover the details of formazans and their metal complexes from the last three years. Therefore, the authors summarize the chemistry and biological potentials of formazans and their complexes from the year 2015 to till date.

1.3 General methods for the formation of formazans

Three synthetic approaches have been investigated for the synthesis of formazan [35]. The first method [35] was reported by Tezcan and Ozbek [36] which is based on the interaction of aryl diazonium salts with aldehyde and phenyl hydrazones in basic media (Scheme 3). On the other hand, the second, introduced by Katritzky et al. [5] was established on integration of aryl diazonium ionic species with reactive methylene groups (Scheme 4). Furthermore, Schiff bases are the fundamental units for the production of pharmaceutically relevant molecules for formazan analogues. Abu Elenien [37] claimed that the formazans were prepared by oxidizing hydrazidines with the appropriate hydrazine substituents (Scheme 5). The first method is the simplest and most practical; however, atoms of carbon substituents are confined to non-aromatic formations, hydrophobicity of the forum, reagent proportion, and set of substituents are all important in this reaction [38].

Scheme 3

Interaction of aryl diazonium salts with aldehyde and phenyl hydrazones in basic media.

Scheme 4

Coupling of aryl diazonium ions with active methylene groups.

Scheme 5

Oxidation of hydrazidines with the appropriate hydrazine derivatives.

2 Literature review

2.1 Synthesis and biological activities of formazans

Formazans are employed in environmental monitoring as chemical and electrochemical sensing devices, as well as test equipment for rapid analysis, and as organic synthesis intermediates. A majorly effective approach for the bio-synthesis of formazans that adheres to conventional green chemistry standards was recently introduced against this backdrop [39, 40].

Lipunova et al. [30] defined the following scheme by a solvent-free mechanism of diazotization of aromatic amines 19 using NaNO₂, nano BF₃·SiO₂ into aryl diazonium salts 20, followed by diazo coupling with 21 at ambient temperature. By cyclizing 22, followed by oxidation via a particularly adapted process, a novel series of 23 was synthesized with verdazyls 23 yielding 88–92%. Under the conditions of the Sonogashira cross-coupling reactions, iodine-containing verdazyls 23 were successfully converted into ethnyl derivatives 24 or 25 for the first-ever (Scheme 6 & 7).

Scheme 6

Synthesis of formazan from substituted amines via NaOH.

Scheme 7

Synthesis of formazan derivatives via THF & TEA.

According to the literature, several formazans have been identified as having a wide range of biological actions and therapeutical applications [41] including antimicrobial [42, 43], antiviral [15, 44], anticancer [18], and anti-inflammatory [45]. The formazan derivative 26 was found to effectively inhibit the illness of Ranikhet virus [46]. In addition, formazans 27 and 28 derived by Misra and Dhar [33] provided high protection against the Ranikhet syndrome. Lakshmi et al.[47] also discovered that various formazans 29 have remarkable antibacterial and antifungal properties. Uraz et al. [48] also synthesized formazan analogues 30 & 31 with several substituents and examined their antifungal effects against a group of microorganisms (Scheme 8).

Scheme 8

Structure of some antibacterial and antifungal formazan compounds.

After that Mahmoud et al. [14] successfully prepared herein a fresh series of nano-sized formazan analogues to investigate their potency as antifungal and antibacterial promoters in vitro. Dithiazone 32 was reacted with a series of ester-hydrazonyl chlorides 33 in the presence of EtONa at normal degrees with vigorous stirring to give product 34. Under similar conditions, another series of formazan analogue 36 has been obtained from the mixture of dithiazone and acetylhydrazonyl chlorides 34. When the reaction was carried out with phenacyl bromide substituents 37 by mixing at room temperature, a formazan derivative 38 is obtained (Scheme 9).

Scheme 9

The reaction of dithiazone with different hydrazonyl chlorides in the presence of EtOH.

Target compounds with an electron withdrawing groups were assessed in vitro for antibacterial and antifungal potential against two gram-positive bacteria, Bacillus subtilis, and Staphylococcus aureus; two gram-negative bacterial strains, Proteus vulgaris and Escherichia coli along with two fungal strains, Aspergillus flavus and Candida albicans using ketoconazole and gentamycin as a source of antibacterial and antifungal stimulants, respectively [49].

Against gram-positive bacteria, the majority of compounds showed moderate to better antibacterial activity. The acetyl derivatives had the best activity against S. aureus and B. subtilis, with inhibition zone values higher than gentamycin. In addition, most compounds appeared to have only minor antibacterial activity against E. coli gram-negative bacteria. The majority of the formazan analogues tested showed promising antifungal activity against the tested fungal strains [50].

In continuation of our previous work of synthesis of nanosized formazan and investigation of their activity against COVD-19, as well as the promising data of these derivatives. Bayazeed et al. [51] interested in synthesizing a new series as an antimicrobial agent. All formazan analogous 41, 43, 45 and 47 were prepared by stirring at room temperature a mixture of the selected hydrazonyl chlorides 40, 42, 44 and 46 and the dithiazone 39 in sodium ethoxide solution (Scheme 10-11). In all cases, only one product was formed as an s-substituted derivative of dithiazone.

Scheme 10

Synthesis of formazan derivatives.

Scheme 11

Synthesis of formazan analogues.

All new formazan compounds were synthesized and tested for antimicrobial activity and MIC values against three gram-negative and three gram-positive bacterial strains, as well as three fungal species. All compounds showed promising antimicrobial screening results, with MIC values ranging from 1.95 to 62.5 g/mL. Compounds 45 and 47 were effective against gram-positive bacteria S. faecalis. Remarkable inhibitory activity was brought by compound 45 against gram-positive bacteria (MRSA). Furthermore, compounds 43b exhibited strong antibacterial potential towards E. coli. Molecules 45a & 47, on the other hand, were extremely effective against K. pneumonia. Compound 43b exhibited antifungal activity against Candid albicans. Although comparable to the reference standard ketoconazole, compound 45 inhibited the three fungal strains significantly. Furthermore, compound 41b outperformed A. niger, C. albicans, and Rhizopus species. Except for compounds 41b, 43a, and 47, all compounds demonstrated remarkable antifungal activity against Rhizopus species.

In continuation, Mehmet et al. [52] synthesized the (o, m, and p-CNF) formazans via coupling reaction. 48 were formed by treating 4-((2-phenylhydrazono)methyl)benzonitrile 50 with NaOH after being mixed in ethanol [53]. 49 was synthesized by reacting (o, m, and p)-nitro anilines with concentrated HCl and NaNO₂. This solution was combined with hydrazine to produce compound (o, m, and p-CNF) 51 (Scheme 12).

Scheme 12

Synthetic route of formation of (o, m, p-CNF).

Fresh formazan signaling pathways with schiff bases nitro units as nitrogen-based hydro donors are designed and synthesized in this regard depending on the features of either the azo or hydrazo groups in the formazan schema [54]. Further, Turkoglu [7] synthesized fresh formazans 53 via coupling mechanism of substituted hydrazines 52 with diazonium salt of p-anisidine (Scheme 13).

Scheme 13

Synthesis of formazan sensors i.e. FNB and FDNB via coupling reaction.

Also, the diazyl formazans 56 were prepared by Habibe et al. [55] (Scheme 14). At 0°C, the produced hydrazone 54 was diluted in pyridine before being added to the solution of sodium hydroxide. Sodium nitrite was saturated and gently added to a solution of p-aminoazobenzene in HCl to get 55. In continuation, 55 were then poured into the hydrazone solution 45. The solid 56 was obtained by filtering, rinsing, and recrystallizing with ethanol [55].

Scheme 14

Synthesis of diazyl formazans.

Here, Mohammed and Dahham [56] synthesized formazan derivatives 61 by combining Schiff base 60 prepared from 1,3-benzoxazol-2-ylhydrazine 59 and different aldehydes with compatible aniline equivalents 57 in the presence of pyridine (Scheme 15) and evaluated their antibacterial efficacy against S. pyogene and E. coli using the agar diffusion method. The results revealed that the compounds having chloro and nitro substituents 60 & 61 were found to be highly active against E. coli and S. pyogene in the tests. The compound having bromo group 61 showed no effect on the bacteria tested [57].

Scheme 15

Pathways for synthesized formazans.

Furthermore, a sequence of new heterocyclic comprising formazan counterparts 65 was synthesized by Begum et al. [19] via the treatment of 4-pyridinaldehyde 62 with 63 in the vicinity of organic acidic catalyst which give Schiff bases 64. In the presence of a base, 64 are coupled with multiple diazonium ions to yield a novel class of formazan compounds 65 (Scheme 16).

Scheme 16

Preparation of formazan compound using organic catalyst.

In addition, Solanki and Marjadi [58] prepared formazan derivatives 69 through a mixture of 4-(4-ethylpiperazin-1-yl) benzenamine 66 and benzaldehyde 67 in absolute alcohol. After that the solid product 68 was obtained. Identically, the leftover Schiff bases 68 as shown in (Scheme 17) were prepared [59]. Further, Schiff bases 68 were treated with a diazonium salt in the presence of dry pyridine. The resultant dark-colored mass 69 was obtained [60] and were screened for antibacterial activity against four bacterial strains namely S. aureus, P. aeruginosa, S. pyogenus, E. coli. Moreover, the antifungal properties of the following formazans were evaluated on the C. albicans. The results of that study indicated that formazan derivative containing electron withdrawing groups displayed prohibitive inhibition against S. aureus. Moreover, compounds containing 4-chloro and 4-methoxy substituent at R₁-position showed moderate activity against S. aureus. Although some formazan derivatives, having electron withdrawing substituents i.e. chloro and methoxy groups at position-4 had excellent activity against C. albicans with MICs of 128 g/ml. Compounds having nitro and methoxy groups at position-2 seemed to have moderate antifungal activity.

Scheme 17

Synthetic route for formazan derivatives.

Recently, Gu et al. [61] originated a mixture of gluconic-meglumine acid as a stimulating vehicle for the reaction of methanal and β-ketosulfones. N-methylglucamine is a potential candidate having physiological inertness and biodegradability. Among other organic compounds, it has recently been investigated as precursors for the synthesis of 1,2-diazoles, pyranopyrazole derivatives, dihydro-pyridines, 2-amino-4H-pyrans, and pyrazolo-pyranopyrimidines [62–65]. Firstly, Khan et al. [66] synthesized the aromatic hydrazone deploying meglumine as a promoter, which then interacted with an aromatic diazonium salt to form formazans. Metal complex formazans were produced using a metallization mechanism with a variety of metal salts and then tested for pharmaceutical applications such as antibacterial, antifungal, and anticancer.

The hydrazine 74 was prepared by reducing of 71 of p-aminobenzenesulphonic acid 70 into phenylhydrazine-4-sulfonic acid which upon condensation with 72 in the involvement of meglumine gives resultant yields 73. The reaction of diazonium ions 75 of compound 74 and synthesized hydrazine 73 produced formazan compound 76 (Scheme 18).

Scheme 18

Reactions pathways for preparation of compounds.

The reported results indicate that the synthesized formazan compounds 76 were tested for antioxidant activity enlisting an adapted-protocol relying on Brand–William’s approach [67–69] as well as for antibacterial actions counter various bacterial species including Staphylococcus epidermidis, Klebsiella pneumonia, Pseudomonas aeruginosa, and Bacillus cereus [70, 71].

In continuation, Turkoglu & Berber [9] produced one more series of compound 82 by conjugating the reactions of 79 with suitable 81 in pyridine (Scheme 19). Formazans 82 were obtained using the first method described by Nineham [5].

Scheme 19

Preparation of formazan compounds.

Furthermore, Samel et al. [12] formed multiple formazan derivatives 86 which were assessed for their antibacterial activity against E. Coli and S. Aureus, as well as their antifungal activity against C. albicans and S. Cerevisiae by using the cup-plate method by coupling 85 prepared from 84 and numerous aldehydes with aryl diazonium chlorides in pyridine (Scheme 20).

Scheme 20

Synthesis of formazans via coupling Schiff bases.

Recently, photodynamic therapy (PDT) against tumor tissues revealed variant promising anticancer activity in formazan derivatives and their conjugates[72]. However, formazans are more practical in terms of determining the efficacy of anti-cancer drugs [73].

All newly formed compounds by Turkoglu & Akkoc [74, 75] have slightly elevated cytotoxic activities in this cell line at various concentrations. Compounds 88 with a fluoro substituent at the para orientation showed increased cytotoxic activity. Comparing compound 88 having the fluoro group at p-position in the 1-phenyl to the other substances tested, showed greater cytotoxic activity. Contrarily, compounds 88 with a bromo group at p-position of a 1-phenyl exhibited the lowest impact of anticancer activity against the breast cancer cell line. Compounds 88 have been shown to have significant substituents on them based on their measured anti-proliferative activities against human cell lines. A diazonium salt solution was prepared by adding an aryl amine 64 to a NaNO₂ solution in the presence of an acid. The diazonium salt of aryl amine solution was then poured dropwise into a mixture of 55 in pyridine. Recrystallization from methanol was used to purify the final product 65 (Scheme 21).

Scheme 21

Preparation of formazan compounds from aryl amines.

Several formazans also showed favorable anti-fertility and anti- parkinsonian activity [76]. Isoniazid (isonicotinic acid hydrazide, INH) has been a prominent anti-tubercular agent for decades, but more information about its action against Mycobacterium tuberculosis has recently emerged [77].

For example, Ayed et al. [78] incorporated a set of formazans via a solution of amine derivatives. The amine derivative 89 was treated with aromatic aldehydes 90 to give phenyl hydrazones 91. Further, the solution of phenyl hydrazone 91 dissolved in dry pyridine. The mixture 91 was then poured into the water with continuous stirring. Then, filtered and dried in order to get product 92 (Scheme 22).

Scheme 22

Synthesis of formazan derivatives.

Also, Al-Faham & Aljamali [79] synthesized formazans as depicted in the following scheme. Compound 94 dissolved in hydrochloric acid with a mixture of sodium nitrite at low temp. Then, compound 95 was added to the composition in the basic medium for three steps. After 24 hours, the formazan compound 96 was filtered and recrystallized [80–84] (Scheme 23).

Scheme 23

Preparation of bis-formazans.

Analyses based on the metabolic lowering of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide has been used to measure virus-induced mutagenic effects [85], and cell viability [86]. MTT reduction in cells results in the generation of a colored, non-soluble formazan compound 97 (Scheme 24).

Scheme 24

Structure of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT).

Over a broad extent of drug concentrations, the standard anti-HTV-1 agents 2’,3’-Dideoxy-ATP (wells B11, C11-B4, and C4) and AZT (wells D11, E11-D4, and E4) promoted cell survival and thus substantial production of XTT formazan in this compound [87].

Recently, Shinde et al. [88] were obtained a series of fluoro substituted formazan derivatives 100 from the salts of diazonium chloride 99 and fluoro schiff base 98 in pyridine (Scheme 25) and were tested for antibacterial efficacy against a variety of bacterium strains such as gram-positive bacteria staphylococcus aureus and gram-negative bacterial strains of Escherichia coli, Klebsiella Pneumonia, Pseudomonas aeruginosa. According to the screening analyses [89], the synthesized formazans derivatives 100 demonstrated excellent antibacterial efficacy against all of the species tested. It was also observed that the electron-rich 3h with three methoxy substituents was likewise cited as having the inhibitory activity.

Scheme 25

Synthesis of fluoro substituted formazan derivatives.

Also, the poison plate technique was being operated to evaluate the antifungal activity of synthesized compounds 100 against the following pathogenic fungi, named as Candida albicans, Penicillium citrinum, Fusarium oxysporum, and Aspergillus niger using ketoconazole as reference drug. The synthesized compounds 100 exhibited remarkable antifungal activity against various organisms, as per the screening studies. The most active compound was the difluorinated compound, which was followed by an electron-rich substrate [88].

Furthermore, Ostrovskaya et al. [90] incorporated tetrazolium betaines having sulfo groups and their precursors. The synthesis of 104 involved three steps: condensation of phenylhydrazine-4-sulfonic acid hemihydrate 101 with aromatic aldehydes to obtain 102, azo coupling of 102 with azobisbenzene to get formazan 103, and oxidation of the 103 with NBS to gain 104. In the following step, we acquire two derivatives formed by the azo coupling of benzene with the –SO₃H substituent; formazan 105, which were segregated from the desired product through separation using CHCl₃. To get 106, oxidation of 105 with NBS had been done. The synthesis of 110 was divided into two phases: azo coupling of 107 with 4-sulfobenzenediazonium to yield formazan 109 (Scheme 26).

Scheme 26

Chemical reactions for 5-(4-nitrophenyl)-3-phenyl-1- (4-sulfophenyl) formazan via cyclization.

2.2 Formazan in dyes

The synthesis of formazan dyes necessitates specific circumstances, such as modest temperatures, use of polar solvents, saturated liquids, fraught with equipment degradation and environmental conflicts Metal formazanates are either preferable as environmentally friendly dyes or as elements of digital recording media, photo and thermo sensitive materials, extremely active catalysts [73, 91].

In 1947, Ciba was the first to make use of metal chelates of tetradentate formazan as wool dyes. Since then, various metal complexes of tetradentate formazans have been identified, and become a widespread variety of colors, particularly in volatile and reactive dyes; although they have also been operated for different purposes [92]. Yuzhen et al. [93] formed a set of five Cu(II) chelates of tetradentate formazans 112 originated via the chromophore 111 (Scheme 27).

Scheme 27

Structure of formazan dye-containing Cu(II) complexes.

Formazan has historically been employed in different application domains. It can, for example, induce very rapid and intense color reactions with a large number of metal ions. They are frequently used as analytic intermediates. Color photography also tends to make use of the reduction of salts containing tetrazolium into metallized formazan pigments. Water-soluble colorless tetrazolium salts have been found to be reducible to water-insoluble deeply colored formazans. Water-soluble complexes with -SO₃H groups have a strong tendency for peptide and polyamide fibers and can be utilized to dye and manufacture make synthetic materials. Acid Black 180 (1) is an example (Scheme 28). The most beneficial dyes of formazan are the 1:1 Co chelated formazan organic dyes used in cellulose fiber coloring and printing [94].

Scheme 28

Structure of formazan dye.

BF₂ complexes of easily available formazan binders are an emanating type of dye with tunable optoelectronic and redox properties. As near-infrared colorants [95, 96], narrow band-gap polymers [97], and cell cytotoxic agents [98], BF₂ formazanates have shown potential. Benzosulfonazole-substituted formazans [99] and their coordination compounds [100, 101] have previously been explored, but derivatives of trifluoroborane were previously unknown.

Further. Chang et al. [102] synthesized boron containing formazan complexes via the condensation reaction of 115 with benzaldehyde was the first step in the production of formazans 116. After combining the generated hydrazone with the suitable aryl salt of diazonium chloride to afford formazans 115 in yields extending from 32 to 87%. Formazans 115 were treated with excess triethylamine and boron trifluoride etherate in methylbenzene at reflux to produce BF₂ formazanates 116 (Scheme 29).

Scheme 29

Synthesis of boron trifluoride formazanate complexes.

When metallized with different metal salts, e.g., hydrated form of ferrous sulphate to generate a complex of ferric ions, chromium chloride to produce Cr(III) ion complex, and copper sulphate to form copper coordination complex, the formazan dyes tend to form metal complexes. The dyes of formazan are also more advantageous in terms of determining the efficacy of anti-cancer medications [103].

Khan et al. [104] prepared this series of metal complexes which were screened for antibacterial potential against various strains such as gram-positive and gram-negative bacteria. The 120 which act as a couple reagents were prepared. After that 121 were synthesized. It was a non-metallic formazan dye that was converted into metal ion complexes with the salts of Fe, Cu, Ni, and Co were distinguished through spectroscopic techniques. The production of un-metallized and metal complex formazan dyes was shown (Scheme 30).

Scheme 30

Preparation of formazan metal complexes.

2.3 Coordination chemistry of formazans

Over the past two decades, interest in the coordination chemistry of formazans and formazanates has been revived due to the simple synthesis and distinctive properties of these complexes. Here, we give an overview of recent research with an emphasis on the reactivity and applications made possible by this special class of ligands. We have opted to arrange our data by periodic table group, starting with alkali metal complexes.

2.3.1 Alkali metals

It has been demonstrated that deprotonation of formazans with highly alkaline metal bases e.g., sodium hydride or potassium hydride generates the corresponding alkali metal formazanate salts. As a result, dimeric, hexameric, and polymeric solid phase systems are formed [105] (Scheme 31).

Scheme 31

Synthesis of Na and K metal salts of formazanate ligands.

2.3.2 Fe-Formazan complexes

Hicks and associates recently reported an iron composite with a trianionic nitrous oxide linker relying on the moiety of formazanate, in which the N-aryl substituents have an auxiliary phenoxide o-donor component. The low-spin Fe(III) cluster [106]was synthesized by salt hydrolysis reaction from the in situ tetradentate dioxyhydrazine generated Na-salt with FeCl₃ (Scheme 32).

Scheme 32

Formation of formazanate scaffold containing iron and cobalt complexes.

The formation of the ferric ion complex with two monoanionic binding sites of triarylformazanate was using salt metathesis and potassium formazanate was reported [107] (Scheme 33).

Scheme 33

Synthesis of formazanate complex 128 and the corresponding by-product 131.

2.3.3 Co-Formazan complexes

Gilroy and Otten [108] reported the formation of the Co(III) multiplex with a trianionic and tetradentate N₂O₂ cyano-formazanate ligand. Complex 134 was gained by salt metathesis with Co(II) or by oxidation of air. Triphenyl formazanate cobalt complexes with one or two semiquinonate (SQ) ligands were reported by Pod and coworkers [109] (Scheme 34).

Scheme 34

Synthesis of formazanate cobalt multiplex with o-quinone based polydentate.

2.3.4 Pd-Formazan complexes

Palladium complexes 136 with formazanate containing nitro group and fluorinated pentane-2,4-dione ligands were prepared [110]. Lipunova et al. [111] described formazanate palladium complexes 135 containing heteroaromatic substituents. The resultant after treatment with ammonium hydroxide was distinguished as a dimeric species 136 using single-crystal X-ray diffraction (Scheme 35).

Scheme 35

Synthetic route for preparation of Pd–Pd bonded complex 139 via the chloride-bridged dimer 138.

2.3.5 Cu-Formazan complexes

The Hicks group outlined [106] the copper-containing formazanate moiety 141 in which each N-aryl side chains have an extra availability of methoxy group (Scheme 36).

Scheme 36

Synthesis of copper containing formazanate complex 141.

2.3.6 Zinc-Formazan complexes

The bis(formazanate) zinc complexes 145 were synthesized by protonolysis and ZnMe₂ (Scheme 37), and their properties were investigated using voltammetry and chemical formulation [112, 113].

Scheme 37

Synthesis of complex 132 and corresponding radical anions 144 and dianions 145.

2.3.7 Boron-Formazan complexes

Hicks and colleagues [114] piqued interest in this family of chemical compounds by converting triphenyl formazans into complexes of tetraacetyl diborate formazanate ligands and demonstrating conversion into verdazyl-type radical anions (Scheme 38).

Scheme 38

Formation of boron complexes and their corresponding radical anions.

An alternative synthesis involves heating toluene solutions containing excessive triethylamine and borane for conversion of 3-cyanoformazan to BF₂ complexes. BF₃.OEt₂ [115] and was later elongated to formazans containing nitro and phenyl groups to yield BF₂ ion complexes [116] (Scheme 39).

Scheme 39

Synthetic route for preparation of boron-containing formazanate ligands.

2.4 Structure activity relationship

From the above table, we observed that group R and group R ₂ exhibited biological activities effectively in comparison to group R ₁ . In group R₁, no notable results have been found to date. This causes a gap in the research, which may encourage young scientists to continue their work in this area.

If R is an aromatic ring containing nitro, methoxy and acetoxy groups at the para position enhance antimicrobial activities [46, 117]. For example, compounds 26, 27, and 30 possess good antimicrobial activities. Similarly, group R having electron withdrawing substituents such as –OCH₃, -Cl, -NO₂, -Br, -N(CH₃)₂ at m- and p-position showed excellent antibacterial and antifungal activities [14, 118].Compounds 34, 36, 38, 92 and 100 are examples. In addition, R is an aromatic ring containing chloro, bromo, fluoro, nitro, and methoxy groups at the p-position increase the antibacterial efficacy of the compounds [55, 57].

If R ₁ is an aromatic ring containing different EWGs inhibited good antiviral activities [119]. Similarly, group R ₁ having –F, -Cl, -Br, and –NO₂substituents at the para position influence anti-proliferative and anti-cancer activities very highly [120]. For example, compounds 88 exhibited these activities effectively.

If R ₂ is an aromatic ring having –NO₂ group showed good antioxidant propensity. Also, group R ₂ having –SO₃H and –Br groups influence antibacterial activities [58, 104]. For example, formazan derivatives 61 & 69 and metal complexes 121 showed excellent antibacterial activities. In addition, aromatic ring R₂ containing electron withdrawing groups such as –Cl, -Br, -OMe at p-position exhibited remarkable antibacterial and antifungal activities [12, 56]. If R ₂ is a heterocyclic ring having chloro group at para position exhibited anti-tubercular activity [121]. Compounds 82 are the example of group R ₂ type. Also, R ₂ is an aromatic ring having –NO₂ and –SO₃H at meta and para positions showed antiviral and anti-HIV activity [85]. Formazan compound 97 demonstrated as an antiviral agent.

Sr. No.	Site	Effect on activity	References	Structure no.
1.	If R is phenyl ring	Phenyl ring substitution gives optimum activity; especially electron withdrawing phenyl rings as compared to electron donating substitution decreases the activity. EWGs enhance the antimicrobial activities.	[14, 88]	26, 27, 30, 34, 36, 38,47, 92, 100
2.	If R is phenyl ring, substitution at para position	EWGs enhance antibacterial and anti-fungal activities.	[51, 56–59]	41, 43, 45,60, 61, 68, 69
3.	If R1 is phenyl ring	Substitution on R1 decreases to some extent but sufficient evidence is available to conclude i.e. EWGs inhibit anti-viral activities	[87]	97
4.	If R1 is phenyl ring, substitution at para position	EWGs influence anti-proliferative and anti-cancer activity.	[75]	88
5.	If R2 is phenyl ring	Highly influence antifungal and antibacterial activities, shows maximum antioxidant propensity and anti-fertility activity.	[59, 104]	69, 76, 121
6.	If R2 is phenyl ring, substitution at para position	EWGs enhance antibacterial activity, anti-viral activity, and anti-HIV. Chloro group inhibits maximum anti-tubercular activity. Optimum results shown by various halogens.	[9, 87]	61, 82, 86, 97

3 Conclusion

After a thorough review of the literature and critical analysis, it was investigated that the effect of a definite type of substitution on a particular position is specific, for example, the electron withdrawing groups (EWGs) on the ortho and para-positions of phenyl rings (R, R₁ & R₂) attached to the formazan group can increase activity to a greater extent. Aside from that, as per the study the electron donating groups (EDGs) in the different positions (R, R₁ & R₂) of the substituted moieties showed no efficient activity. From SAR studies, the substitution of various substituents on –NH moiety decreases the efficiency of the biological activities. It is hoped that this review would pique the interest of other research groups, encouraging them to develop additional formazans that are more efficacious.